Cracking catalysts based on rare earth metal exchanged faujasitic zeolites are well known in the art. These rare earth metal exchanged zeolites generally exhibit high activity and stability and are being preferentially employed in fluid catalytic cracking (FCC) processes. These properties are obtained by substituting some of its sodium content with other metals, hydrogen, or ammonium. Further enhancement of FCC properties can be achieved by rearrangement of the zeolite structure by high temperature treatment of the zeolite with steam which will cause a portion of the framework aluminum to migrate into nonframework positions. Removal of the aluminum from framework positions to nonframework positions produces an ultrastable zeolite of high thermal and hydrothermal stability. These nonframework aluminum atoms can then be removed from the crystalline structure by treatment with caustic, acids, such as HCL, or with salts, for example, Na.sub.2 H.sub.2 EDTA, thereby resulting in an increased silica-to-alumina ratio preferred in fluid catalytic cracking operations.
Preparations of dealuminated zeolites of the faujasite type have already been described in several publications. Thus, an article in the Journal of Catalysis, 54, pp. 285-288 (1978) by J. Scherzer provides a detailed discussion of methods involving the preparation of highly stable dealuminated zeolites. Also, in U.S. Pat. No. 4,218,307 (McDaniel), dealumination of faujasitic zeolite using hydrothermal treatment and mineral acids is disclosed.
In most of these prior art methods, dealumination is accomplished by the use of chemicals which may damage the crystal structure of the treated zeolite, and thus reduce the stability of the zeolite. Application of EDTA for dealumination is a milder method for aluminum removal; however, from an economic point of view, it is impractical due to its relatively high cost and its limited ability to remove nonframework aluminum.
It has now been found that acid-type ion exchange resins can readily accomplish the dealumination without detrimentally affecting the structural strength of the exchanged zeolite. In addition, the treatment is simple and economic, resulting in a dealuminated zeolite of high activity and stability, low coke-forming properties, and a high silica-to-alumina ratio.